A D1 format component type digital VTR and a D2 format composite type digital VTR both of which are used in broadcasting stations have been practically used as digital VTRs for digitalizing color video signals and recording them on a recording medium such as a magnetic tape.
In the former D1 format digital VTR, a luminance signal and first and second color difference signals are A/D converted with sampling frequencies of 13.5 MHz and 6.75 MHz, respectively. Thereafter, the signals are processed in a particular manner and then recorded on a tape. Since the ratio of sampling frequencies of components is 4:2:2, this system is also named the 4:2:2 system.
On the other hand, in the latter D2 format video digital VTR, a composite video signal is sampled with a signal with a frequency 4 times higher than the frequency fsc of a color subcarrier signal and then A/D converted. Thereafter, the resultant signal is processed in a particular manner and then recorded on a magnetic tape.
Since these VTRs are designed for use in broadcasting stations, the top priority of them is in picture quality. In these VTRs, a digital color video signal where one sample is A/D converted into for example 8 bits is recorded without substantially being compressed.
As an example, the data amount of the former D1 format digital VTR will be described in the following.
When each sample is A/D converted into 8 bits with the above mentioned frequency, the information amount of a color video signal is approximately 216 Mbps (megabits per second). When the data in the horizontal an vertical blanking intervals are removed, the number of effective picture elements of the luminance signal per horizontal interval and the number of effective picture elements of each color difference signal per horizontal interval become 720 and 360, respectively. Since the number of effective scanning lines for each field in the NTSC system (525/60) is 250, the data amount Dv of picture signal per second can be expressed as follows. EQU Dv=(720+360+360).times.8.times.250.times.60=172.8 Mbps
Even in the PAL system (625/50), since the number of effective scanning lines for each field is 300 and the number of fields per second is 50, it is obvious that the data amount in the PAL system is the same as that in the NTSC system. By considering the redundant components necessary for error correction and format with respect to such data, the bit rate of picture data becomes approximately 205.8 Mbps in total.
In addition, the data amount of audio data Da is approximately 12.8 Mbps while the additional data Do such as a gap, a preamble, and a postamble used in editing is approximately 6.6 Mbps. Thus, the information amount of overall record data can be expressed by the following equation. ##EQU1##
To record the above mentioned information amount, with respect to the D1 format digital VTR, a segment system having a track pattern with fields of 10 tracks in the NTSC system, and with fields of 12 tracks in the PAL system has been employed.
In addition, a recording tape with a width of 19 mm is used. There are two types of recording tapes in thickness, which are of 13 .mu.m type and 16 .mu.m type. To house these tapes, there are three types of cassettes, which are of large type (L), middle type (M), and small type (S). The information data is recorded on such tapes in the above mentioned format. Thus, the data recording density is approximately 20.4 .mu.m.sup.2 /bit. When the recording density is high, an error tends to take place in the playback output data due to interference between codes or non-linearity of electromagnetic conversion system of the head and tape. Thus far, even if the error correction encoding was performed, the above mentioned value was the limit as the recording density.
By putting all the above mentioned parameters together, the playback time of the cassette in each size of the digital VTR in the D1 format can be tabulated as follows.
______________________________________ Size/tape thickness thick 13 .mu.m thick 16 .mu.m ______________________________________ S 13 minutes 11 minutes M 42 minutes 34 minutes L 94 minutes 76 minutes ______________________________________
As was described above, although the D1 format digital VTR can satisfactorily provide high picture quality for use in broadcasting stations, even if a large cassette housing a tape with a thickness of 19 mm is used, the playback time is at most 1.5 hours. Thus, such a VTR cannot be adequately used for a home use VTR. In order to increase the data recording density, it is effective to reduce the track pitch. However, in the conventional VTRs, error in reproducing data increase because of nonlinearity of track, tracking error, etc., so that it is limited to reduce the track pitch. In particular, it is difficult to guarantee the compatibility among VTRs.
On the other hand, as home use VTRs, the .beta. system, the VHS system, the 8-mm system, and so forth have been practically used. However, in each of them, data is recorded and playback with analog signals. Thus, although their picture qualities have been satisfactorily improved, for example, when data which was imaged and recorded with a camera is dubbed and copied, the picture quality is remarkably degraded. Thus, when the data is dubbed several times, the picture quality will become intolerable to the viewers.